Precooling and purification of gaseous mixtures prior to liquefaction



May 29, l951 P. M. scHUl-TAN 2,555,060

PREcooLING AND PURIFICATION oF GAsEoUs MIxTuREs PRIOR To LIQUEFACTIONFiled May 14, 1947 25 Ay A x/L/Am 1% r/ eEFK/GEKANT Z' m OUT N/ Tgo GENFEA CTM/v7 m I TGA 5501/6 SEPARAT/Na [D C0 oLAN-r I I l UNIT I ourMAME-UP /2 FLUID /N PZECOOLED All? 007' INVENTQRV 24 f/ BAULNAuff/cfSf//UFTAN ATTORNEY Patented May 29, 195i UNITED S TAT-'ES PATENTOFFICE' PREGOOI-INGA AND PUEIFICATION OF GAS- EOUS MIXTURES l.PRIOR T0LIQUEFAC- TION The present invention relates to the 'precooling andpurification f gaseous mixtures ,p'r'ir to liquefaction.

In what follows the Word impurities vn'i'ustbe understood to embracealsoan impurity in' the singular.

The term fprecooling'properly refers" to the ,precooling of the gaseousmixture by means of an` lauxiliary refrigerant, whereby -the overallthermodynamic efliciencyof a liquefaction cycle employing .YJoule-Thompson expansion is Vmuch improved. Infpractice, however theterm""p'r'e cooling conveniently refers to the whole of the cooling ofthe gas performed before it proceeds to the liquefaction unit proper,which' usually includes also some cooling obtained from one or more ofthe cold gaseous products from 'the 'sequent liquefactionprocess.Precoolingiin `this sense istherefore usually performed in two'steps,

4 butmay of course in princple'be performed solely .sufficiently toensureV prolonged operation of the liquefaction un-it proper before itsconstruction byvsoliddeposits of these impurities. AExamples ofimpurities of the type referred to are waterin air,4 and benzeneand/-ornaphthalene in carbonisation gases. Constituents not originally presentin the gaseous mixture, such as oil vapours from a compressor-lubricant,which would -also obstructthe liquefaction unit, and in an airseparation unit might cause a danger of explosion, will alsobe removedto a considerable extent in the precoolingprocess.

Precooling as. carried out. hitherto has suffered fromsseveraldisadvantages. In normal practice agaseousmixture is precooled bybringing it into direct/heat Vexchange through `the walls fof a thermalconductor with -gaseous products from -the sequentliquefaction processand/or with an auxiliary refrigerant. Owing to the depositionv ofimpuritiesvinthe solid state, theiprecoolers become .obstructed in -arelatively short time and are ytherefore usually provided in duplicateso thatone unit maybe thawed Aout whilst the otherunit-.is.performingitsV function as a cooler. In view ofthe necessity.of securing a reasonable time` of operation before complete obstructionof the gas-passages, itis-necessary for these 'pas- Y sages to be ofrelatively large cross-sectional ar'a `from being undesirablein*-themselvesfcausa larger amountVV of the impurities V'with'relatively high vapourpressureand relatively high freezing .point to'reach the 'separation unit thanwouldbe the case if the temperatures andpressuresfirema'ined steady.

Afurther disadvantage'is the: necessity of'providing several'changeovervalves whichiare 'prone to leakage. Theleak'age of even a small amountof warm 'gas through avalve in theclosed position intoa gas stream whichhasbeen precoo'led causes formation of very finely-divided solidparticles Whichmay be Vcarried far into the liquefaction `unit and causeearly obstructions" therein.

It is an bject of this invention '1:0 ensure a-'continuous precoolingoperation Tin a 'simpledev'ice in which pressure losses andcoldlossesar'e' both uniformly Vlow whereby asavinginpovv'erc'onsumption together with improved 4sileatliness 'andease of operation may be'achieve'd.

According to this'inventionv there is provided a `method of pre'coolinga Agaseous mixture having an impurity lcontentl of relatively highvapour `pressure and vrelatively 4high Yfreezing' pointes a preliminaryto liquefying atl leastv ajpartof Lthe Amixture wherein the precoolingis effeeted'bycontacting the' mixture in` counter-curre'ntA flow 'with acold liquid'me'diimof low freezing pointadapted to cool the gas 'mixturearid to retain'suchfa proportion of theiimpurity content that thejvapourpressu're'of the residual impurities inl'th'e 'cooled gaseous mixture issof low as toV ensure extended voperationof the plant in which theseq'uentlliquefaction `is effected. ,Dependingy upon'the natureof theimpurities to be removed vand AtheI temperature, the`mass` transfer mayinvolve solutionin-the liquid'medium, or liq'uefaction `and'thef-for'mation of' an `emulsion in the liquid medium,-orfsolidi-eation and the formation of a susperfsinbf fsolidparticles'in'v the' liquid f mediuiri of thefl impurities. Usually,"however one impuritywill predominate '(e. g; watervapour' in"-air)-=andin sucha caisefthe liquid medium will be chosen so as to dissolve thisimpurity. It is an advantage of this invention that the vapour pressureof a soluble impurity will be reduced by the presence of the liquidmedium, quite apart from the effect of the low temperature.

The contacting of the gaseous mixture with the cold liquid medium may beeifected in any suitable contacting device such as a scrubber or apacked or plate column. The liquid medium at the required lowtemperature is introduced at or near the top of the device and as ittravels down the column in counter-current to the upwardly ovving gasstream, both heatand mass-transfer from the gaseous mixture to theliquid medium occur. As a result of the heat transfer, under eflicientoperating conditions the liquid will leave the contacting device at atemperature only slightly below the inlet temperature of the gaseousmixture.

After contacting the gaseous mixture, the liquid medium may be recooledfor re-use and prior to re-use may be subjected to a purification treat-Y ment for a reduction of the impurity content.

Where the impurities are in solution they may be removed bydistillation, crystallisation or solvent extraction; where they are inthe immiscible liquid state they may be removed by decantation, andwhere they are in the solid state they may be removed by ltration.

Where one or more gaseous products is or are available from the sequentlquefaction, such gaseous product or products or part thereof(hereinafter termed "the gaseous coolant, which term is intended tocover both gaseous products which are to be conserved as valuableproducts of the liquefaction process and those which are to be rejectedas waste products) may be used to recool the liquid medium, additionalcold being supplied if necessary from an auxiliary refrigerating cycle.The liquid medium may, for instance, be recooled by bringing it intoindirect heat exchange with the gaseous coolant through the wall of a.thermal conductor.

Alternatively, the liquid medium may be recooled by directcounter-current contact with the gaseous coolant, additional cold beingsupplied if necessary from an auxiliary refrigerating cycle.

When such direct contact recooling is adopted it is of advantage toretain in the liquid medium up to the stage when it is contacted withthe gaseous coolant at least a part of those impurities having a vapourpressure higher than that of any other constituent or constituents ofthe liquid medium whereby the cooling produced by heat transfer betweenthe gaseous coolant and the liquid medium is augmented by cold producedby mass A transfer of impurity to the gaseous coolant.

Where the impurity to be removed is a substance which is present as aninitial constituent of the liquid medium and has a vapour pressure thosecases where the vaporisation of the im-V purity into the gaseous coolantis not objectionable (as, for instance, When the impurity is of no valueand when there is no objection to contamination of the gaseous coolantby it), for firstly the necessity for a separate apparatus to remove theimpurity from the liquid medium is avoided and secondly additionalcooling of the liquid medium is produced by the evaporation of theimpurity. This Will result in a reduction in the cold productionrequired and therefore in the power consumption of the plant. Forinstance, in an air separation process where water vapour is an impurityto be removed and a glycol-water solution is used as the liquid medium,the latter may be at least partly recooled by bringing it intocounter-current contact with the nitrogen fraction as the gaseouscoolant whereby an appreciable amount of extra cooling of the liquidmedium will normally be obtained as a result of water evaporation intothe nitrogen stream. This extra cooling will reduce the powerconsumption of the separation plant or alternatively that of theauxiliary refrigeration cycle if such is used to provide iinal recoolingof the liquid medium.

Where the amount of the substance evaporated from the liquid mediumexceeds that removed as impurity from the gaseous mixture, it isnecessary, as stated above, to add to the liquid medium a quantity ofthe substance equal to the excess evaporated so as to restore the liquidmedium to its original composition. The addition may conveniently bemade by bringing the substance to be added into intimate contact withthe eiuent gaseous coolant so as to strip therefrom any valuablecomponent of the liquid medium which may have been vaporised by thegaseous product. For instance, when a glycol-water solution is used asthe liquid medium to remove water vapour from compressed air, the wateradded to compensate for the excess evaporated over that removed from thecompressed air can be used to strip glycol vapour from the emerginggaseous coolant.

The choice of the liquid medium will depend on the precoolingtemperature to be achieved and on the nature of the impurities to beremoved from the gaseous mixture and will also be inuenced by particularconditions obtaining in the plant under consideration. Depending on thenature of the impurities to be removed, liquid media Which could be usedare, for example, hydrocarbons, glycerol and other polyhydric alcohols,both aliphatic and aromatic, monohydric alcohols, acetones, aldehydes,organic acids, or liquids of 10W freezing point composed of inorganicand/or organic substances dissolved in wa ter.

In order to avoid excessive losses of valuable substances contained insuch liquid media, it will be of advantage to use ingredients having thelowest possible vapour pressure compatible with the low freezing pointof the liquid.

In the specic case of the precooling of air prior to liquefaction andseparation into a nitrogen fraction and an oxygen-fraction where watervapour is present in the air as an impurity liable to choke theliquefaction and separation units, the liquid medium may be aglycol-water solution. When using the eutectic composition containingabout 58% by weight of glycol and 42% by weight of water, it is possibleto vobtain precooling temperatures approaching minus 49 C. At such lowtemperatures the vapour pressure of the residual water content in theair emerging from the pre-cooler Will be so low that the liquefactionand separating units may opern ate for prolonged periods before havingto be thawed out on account of choking with4 ice. The water taken upfrom theair by the glycol/water mixture may be removed by any knownmeans, such as revaporisation or kcrystallisation, so as to enable theliquid medium to be re-used. It will be of' particular advantage to usefor this purpose one of the productsof separation, the nitrogenrfraction or the oxygen fraction, both of which are substantially dryVand available at low temperature, so that vcooling and water removalare effected simultaneously as already explained in more general terms.

The invention will now be described in further detail with reference tothe accompanying drawing, which shows diagrammatically in Figure i oneform of apparatussuitable for use in carry ing out the invention,employing indirect recooling of the precooling liquid; and-Figure 2shows diagrammatically another form of apparatus employing directrecooling.

A compressed gaseous mixture to be precooled, and having an impuritycontent of relatively high Vapour pressure and relatively high freezingpoint, is led from a compressor (not shown) to the bottom of acounter-current contact unit IB through an inlet pipe IE. The contactunit l may comprise a packed column or a plate column or other suitablecontact device. The air passes upwardly through the unit It! incounter-current contact with a descending stream of precoolingfluid'consisting of a glycol-water solution, and the purified air leavesthe top of the contact unit through outlet pipe I2 and passes to aseparating unit I4 in which the nitrogen fraction is separated andpassed through pipe I5 for use as a gaseous coolant.

The precooling liquid enters the top of the unit it through pipe I3, andafter absorbing the iml purities from the gaseous mixture being treated,passes from the bottom of the unit I through pipe I8 to the top of aheat exchanger Il, in which it is brought into heat exchange relationeship with the gaseous products of separation er1- tering the exchangerI'I through pipe I5 and pipe I8 and leaving through pipe I9. From theexchanger II, the precooling fluid is led through pipe 2) to acirculating pump 2| and thence through pipe 22 to a second heatexchanger 23, where it is passed in indirect heat exchange with anauxiliary refrigerant which enters exchanger 23 through pipe 24 andleaves it through pipe 25. From the exchanger 23, the re-cooled fluid isconveyed through pipe I3 to the top of the contact unit IIi.

The heat exchanger II may be of the type in which heat transfer takesplace through the wall of a thermal conductor, for example, it may be atubular heat exchanger, as shown in Figure 1; in this case water vaporcondensed from the glycol-water solution during its passage through theexchanger II is removed through a drain pipe 26. It may on the otherhand be a direct contact heat exchanger or scrubber similar to the unitEB, as shown in Figure 2. In the latter case, the volume of gaseouscoolant passing through the exchanger I'I may be so great as to removemore water from the glycol-water solution than the latter has absorbedfrom the air. The glycol-water solution is in this case restored to itsoriginal composition by the addition of make-up water. Make-up water issupplied as indicated at 21 in Figure 2.

I claim:

1. The method of precooling air containing water vapour as a preliminaryto liquefying at least part of the air and separating therefrom anitrogen fraction, said method comprising bringing the air into directcontact and in counter-current flow with a glycol-water solution at atemperature below 0 C. adapted to cool the air and to retain such aproportion of the Water content that the partial vapour pressure of anyresidual water in the cooled airis so low as to ensure Vextendedoperation of the plant in which the sequent liquefaction is effected,and restoring the glycol-water solution to a condition suitable forre-use by a regenerationprocess which comprises bringing it into directcontact with a gaseous coolant derived from the sequent liquefaction inorder simultaneously partially to re-cool the glycol-water solution andto reduce the water content1 thereof, additional cold being supplied `tothe glycol-water solution to complete the re-cooling thereof from anauxiliary refrigerating cycle.

2. `The method of precooling airr containing water vapour as apreliminary to liquefying at least part ofthe air and separatingtherefrom a nitrogen fraction, said method comprising bringing the airinto direct contact and in counter current ow with a glycol-watersolution at a temperature below 0 C. adapted to cool the air and toretain such a proportion of the water content that the partial vapourpressure of any residual water in the cooled air is so low as to ensureextended operation of the plant in which the sequent liquefaction iseffected, and restoring the glycol-water solution to a conditionsuitable for re-use by a regeneration process which comprises bringingit into direct contact with a gaseous coolant derived from the sequentliquefaction in order simultaneously to re-cool the glycol-watersolution and to reduce the Water content thereof, the volume of gaseouscoolant being such that the water removed during the regenerationprocess exceeds that absorbed by the glycol-water solution and restoringthe glycol-water solution to its original composition by adding waterthereto.

3. The method of precooling air containing water vapour as preliminaryto liquefying at least part of the air and separating therefrom anitrogen fraction, said method comprising bringing the air into directcontact and in counter current ow with a glycol-water solution at atemperature below 0o C. adapted to cool the air and to retain such aproportion' of the water content that the partial vapour pressure of anyresidual water in the cooled air is so low as to ensure extendedoperation of the plant in which the sequent liquefaction is effected,and restoring the glycol-water solution to a condition suitn able forre-use by a regeneration process which comprises bringing it into directcontact with a gaseous coolant derived from the sequent liquefaction inorder simultaneously to re-cool the glycol-water solution and to reducethe water content thereof, the volume of gaseous coolant being such thatthe water removed during the regeneration process exceeds that absorbedby the glycol-water solution, and restoring the glycol-water solution toits original composition by adding water thereto, the added Water beingbrought into intimate contact with the effluent gaseous coolant so as tostrip therefrom and return to the glycol-Water solution any glycolentrained by the gaseous coolant.

4. The method of precooling air containing water vapour as a preliminaryto liquefying at least part of the air and separating therefrom anitrogen fraction, said method comprising bringing the air into directcontact and in counter current flow with a glycol-water solution at atemperature below 0 C. adapted to cool the air and to retain such aproportion of the water content that the partial vapour pressure of anyresidual water in the cooled air is so low as to ensure extendedoperation of the plant in which the sequent liquefaction is effected,and restoring the glycol-water solution to a condition suitable forre-use by a regeneration process which comprises re-cooling theglycol-water solution with its augmented water content by bringing intodirect contact therewith the separated nitrogen fraction in a volumeexceeding that required to vapourise from the glycol-water solution thatquantity of water which was removed from the air by the originalglycol-water solution, and restoring the glycol-water solution to itsoriginal composition by adding thereto an amount of water equal to theexcess evaporated by said nitrogen fraction, said added water beingbrought into intimate Contact with the effluent nitrogen fraction so asto strip therefrom any glycol entrained therein.

PAUL MAURICE SCHUFTAN.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,501,415 Lafferty July 15, 19241,724,513 Pollitzer Aug. 13, 1929 1,791,086 Sperr Feb. 3, 1931 2,093,805De Baufre Sept. 21. 1937 2,134,699 Brewster Nov. 1, 1938 2,141,997 Lindeet a1. Dec. 27, 1938 2,198,142 Wade Apr. 23, 1940 I2,214,678 RaigorodskySept. 10, 1940 2,245,028 Farris June 10, 1941 2,288,461 Keith et al June30, 1942 FOREIGN PATENTS Number Country Date 591,095 France June 27,1925

1. THE METHOD OF PRECOOLING AIR CONTAINING WATER VAPOUR AS A PRELIMINARY TO LIQUEFYING AT LEAST PART OF THE AIR AND SEPARATING THEREFROM A NITROGEN FRACTION, SAID METHOD COMPRISING BRINGING THE AIR INTO DIRECT CONTACT AND IN COUNTER-CURRENT FLOW WITH A GLYCOL-WATER SOLUTION AT A TEMPERATURE BELOW 0* C. ADAPTED TO COOL THE AIR AND TO RETAIN SUCH A PROPORTION OF THE WATER CONTENT THAT THE PARTIAL VAPOUR PRESSURE OF ANY RESIDUAL WATER IN THE COOLED AIR IS SO LOW AS TO ENSURE EXTENDED OPERATION OF THE PLANT IN WHICH THE SEQUENT LIQUEFACTION IS EFFECTED, AND RESTORING THE GLYCOL-WATER SOLUTION TO A CONDITION SUITABLE FOR RE-USE BY A REGENERATION PROCESS WHICH COMPRISES BRINGING IT INTO DIRECT CONTACT WITH A GASEOUS COOLANT DERIVED FROM THE SEQUENT LIQUEFAC- 